The present invention relates generally to threaded fasteners and more particularly to fasteners which are utilized to extrude and form threads in sheet metal material.
Heretofore, screws have been provided with conical-type, threaded points designated as gimlet points and typically have spaced threads on the point and shank for use in sheet metal applications. Other forms of extruding and/or thread-forming screws utilize a particularly configured blank, such as trilobular in cross section, upon which a thread form is impressed.
Still other varieties of extruding screws have been developed including those which incorporate a thread-forming region intermediate a pilot point and a threaded region with the thread forming region including a plurality of partially formed threads on a tapered shank region and on generally one peripheral segment of the thread-forming region.
While all of the above noted prior art threaded fasteners have met with a certain amount of success, total effectiveness and efficiency of such fasteners still can be greatly improved. For example, the ratio between drive torque and strip torque must be maximized especially as the panel thickness becomes smaller. For a given application, the torque to drive should ideally be very low and the torque to strip should be relatively high.
Many prior art fasteners require a relatively high amount of axial pressure to create the required extrusion and thread formation in an effort to maximize the strip/drive ratio.
Accordingly, a primary object of the invention is to provide a thread-forming and extruding screw which creates a maximum extrusion in a thin panel with a minimum amount of axial pressure.
Still another object of the invention is to provide a thread-forming screw which maximizes the torque to strip and drive ratio.
Still a further object of the invention is to provide a particularly configured blank upon which thread forms can be impressed to create an efficiently performing extruding and thread-forming screw.
A particular advantage of the invention is the provision of a thread-forming region which gradually tapers into the holding region while providing a large number of material working partial threads.
The above and other objects of the invention are provided by a blank and a threaded fastener produced by such a blank which includes a lead region and an upper shank region concentric to one another interconnected by a thread-forming region having a pair of dissimilar peripheral segments. At least one peripheral segment is tapered at an angle of approximately 6.degree. to the upper shank region with the lower region of the one segment extending axially downwardly relative to the lower region of opposing peripheral segment. The thread-forming region is also configured so that any axial location on the thread-forming region the one peripheral segment will be of a greater radial extent from the axis than the other.
A threaded fastener formed on such a blank will have a generally conical or gimlet threaded point merging into the thread-forming region. The thread-forming region will have a plurality of partially formed threads tapering at an angle of about 6.degree. formed on the peripheral segment and which all extend radially from the axis a greater amount than the opposing peripheral segment. The opposing peripheral segment will be substantially void of threads creating a wiping region for reforming and reworking the sheet metal material. All of the above features contribute to an efficient and effective extruding screw. The slight tapering of the thread-forming regions to the threaded shank and threaded-point region reduces the axial pressure necessary to drive and permits the formation of a plurality of pitches of partially formed threads creating a gradually expanding, single lobular region.